Flash Notifications are used to give Flash Alerts when you get a Call / SMS.

This feature is already there is most phones by going to:

Settings > Accessibility > Hearing > Flash Alerts

Toggle there to turn it ON/OFF.

This is really beneficial in alerting without making any sound.

Don't get dishearted if your phone does not have this feature. There are a number of apps available on Play Store.

I'll personally recommend you to use the app named Flash Notification available in play store. It is really easy to use and you can enable it for particular apps and for different sound profiles(Ring/Vibrate/DND).

You can also watch below video to help you activate the default feature:

Waterdrop Notch means The phone has full vision display with minimal bezels and only display, there is just small water drop shape on top that features a camera and other required essential hardware features.

no you can't.. you may find many one about how to bootloader and all they all are just copy pasted material. oppo f9 can't be till this date because oppo didn't launch it's kernel code to publicand oppo uses ozip encryption then there is no other options left if someday oppo launch it's kernel source code then you might be able to root as well as unlock bootloader too